Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a first image of a display screen or portion thereof with a transitional graphical user interface showing our new design;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof; and,

FIG. 4 is a front view of a fourth image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-4. The process or period in which an image transitions to another forms no part of the claimed design.

The broken line showing the display screen or portion thereof, and text, form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 